The so-called SMART CUT® process, illustrated in FIG. 1 provides high quality silicon on insulator (SOI) substrates. During this process (FIG. 1a), two substrates, called a handle substrate 101 and a donor substrate 103, usually Silicon wafers, undergo a certain number of process steps to transfer a layer with a given thickness of the donor substrate 103 onto the handle substrate 101. During the process, the donor substrate 103 is typically oxidized 105 to later on form the buried oxide layer (BOX) of the SOI structure, and an ion implantation step, during which ions like hydrogen or rare gas ions (He, Ar, . . . ) are implanted into the donor substrate 103, is applied to form a predetermined splitting area 107 defining the to-be-transferred layer. Subsequently (FIG. 1b), the source substrate 103 is attached to the handle substrate 101, in particular via bonding taking advantage of Van der Waal's forces, to obtain a source-handle compound 109. Upon a mechanical and/or thermal treatment, a detachment of a semiconductor layer 111 together with a buried oxide layer 113 occurs at the predetermined splitting area 107 so that the two layers are transferred onto the handle substrate 101 to obtain the desired silicon on insulator structure 115 (FIG. 1c). The thickness of the layer 111 is determined by the energy of the implanted ions.
The remaining part 117 of the donor substrate 101, also called the negative, can be recycled and again used in the SMART CUT™ type process as a new donor or handle substrate. The SMART CUT™ type SOI fabrication process has a significant economic advantage due to this recycling process. Indeed, the process provides an optimized use of the raw material for instance silicon wafers.
The negative 117 (FIG. 1c) has a characteristic topography representing protruding residues 119a and 119b in an edge region, as illustrated in FIG. 1, which corresponds to a region where no layer transfer occurred due to the chamfered shape of the edge of the initial wafers 103 and/or 101. The surface of the negative 117 between the protruding residues 119a and 119b has a first inner region 121 at which detachment occurred to provide the transferred layer 111 on the handle substrate 109 and which has a rather rough surface typically close to 60 Å to 70 Å RMS as measured by atomic force microscopy (AFM), which is to be compared to 1 Å to 3 Å for standard Silicon wafer. The edge of the remainder 117 with the protruding residues 119a and 119b actually has a chamfered shape and furthermore comprises a step-like structure 123 seen from the internal region 121 comprising the remaining part of the buried oxide layer 125 and the non-transferred silicon 127 over the remaining part of the ion implanted predetermined splitting area 129. The edge 131 and the backside 133 of the negative 117 are, also covered by the oxide.
The step 123 of the negative 117 typically has a thickness of about 100 Å to 10,000 Å of silicon, mostly between 1000 Å to 3000 Å, and 100 Å to 10,000 Å of silicon oxide and has a width w in the lateral direction of the order of 0.5 mm to 3 mm.
Prior to the reuse of the negative 117 as donor substrate 103 or handle substrate 101, the surface roughness of the inner region 121 needs to be reduced and the protruding residual topography 119a and 119b needs to be removed. The removal needs to be complete as any remaining protruding material can create particle contamination when during a thermal treatment an exfoliation in the chamfered region occurs due the presence of the remaining ion implanted region 129. Methods to do so are, for example, known from EP 1 156 531 A1 and U.S. Pat. No. 7,402,520 B2. Typically, the following process is applied to get rid of the protruding residual topography: The reclaiming process of negative 117 starts with a de-oxidation step to remove the oxide layer 125 on top of the protruding residual topography on the edge of the remainder 117 as well as on the side 131 and on its backside 133. The de-oxidation can, for example, be carried out using a HF bath, wherein the acid consumes the oxide layers 125, 131 and 133. Subsequently, a first polishing step of the edge region of substrate 1 is carried out to at least partially remove the protruding silicon part 127 on the edge. Then a double-sided polishing (DSP) step is carried out to improve the surface roughness in the interior region 121 but also to further remove the step 123 in the direction of the protruding residual topography 119a and 119b, but also to remove residues remaining form the ion implantation. Finally, to obtain a suitable surface roughness on the front surface of the remainder 117, a chemical mechanical polishing step (CMP) is carried out.
Even though, it is possible to obtain a recycled substrate with the described reclaiming process and which can be reused in the SMART CUT™ process, it is an object of the present invention to provide an improved and more economic reclaiming process that no longer needs the double-sided polishing step to reclaim the remainder of the donor substrate. Indeed, the DSP process step has the major disadvantage that, during polishing, up to 10 μm (5 μm on each side of the substrate) of material are removed to get rid of the protruding residual topography 119a and 119b. 
This object is achieved with the method according to claim 1. Accordingly, the method comprises the steps of a) providing a donor substrate, in particular a semiconductor substrate, and a handle substrate each with chamfered edge regions, b) forming a predetermined splitting area at a depth h inside the donor substrate, c) attaching, in particular by bonding, the donor and the handle substrate to obtain a donor-handle compound, wherein no attaching occurs between the two substrates in the chamfered edge regions of the donor and handle substrates, then d) etching the chamfered region such that at least a layer of about the thickness h is removed from the donor substrate in the region where no attaching occurred, then e) detaching a remainder of the donor substrate from the donor handle substrate, wherein detachment occurs at the predetermined splitting area and f) reusing the remainder of the donor substrate, in particular after the surface treatment step. According to a preferred embodiment, during step d), a layer with a thickness of more than h is removed.
Thus, unlike in the process of the prior art, the remainder of the donor substrate will no longer present a protruding portion 119a, 119b as illustrated in FIG. 1, as the chamfered area where no attachment occurs between the two attached substrates is removed prior to the detachment. By the fact that, no longer any protruding regions are present at the surface of the remainder of the donor substrate, there is no need for carrying out the edge removal step and the double-sided polishing step, which were mandatory in the prior art to get rid of the protruding portion. As a consequence, the reclaiming of the remainder of the donor substrate can be carried out in a much simpler way not needing expensive additional tools to carry out the edge polishing and the double-sided polishing process steps. At the same time, due to the suppression of the double-sided polishing step, the initial geometry of the donor substrates can be kept even if one donor substrate is used several times in a process as described above. In addition, due to the fact that the implanted region in the chamfered region is removed during step d), the quality of the obtained substrates is also improved as the risk of creation of particles during a raise in temperature leading to a partial removal of the chamfered region, can be reduced.
Preferably, the method can use a donor substrate comprising a dielectric layer and, in this case, can furthermore comprise a step g) carried out between steps c) and d) which consists in removing the dielectric from the donor substrate at least in the chamfered region but not in the attached region. Thus, even in the presence of a dielectric used to form a semiconductor on insulator substrate with the mentioned process, it is still possible to achieve the advantages of the method as, not only the chamfered region of the donor substrate itself but also of its dielectric layer, is removed prior to detaching.
It has to be pointed out that the removal step to remove the dielectric layer does not represent an additional step which has to be carried out. Indeed, in the prior art, the dielectric layer is also removed during reclaiming. Thus, compared to the prior art, step g) is simply moved from after detachment to prior to detachment.
According to a variant, the handle substrate can comprise a dielectric layer. In this case, the dielectric layer is provided by the handle substrate to form a semiconductor on insulator substrate. This variant has the advantage that one only has to remove the material of the substrate in step d) to achieve the advantages of the invention, as no additional dielectric layer is provided on the donor substrate. Thus, in this variant, less process steps are necessary compared to the variant which provides the dielectric via the donor substrate.
Advantageously, the dielectric layer can be an oxide, in particular a silicon oxide. This oxide layer can be provided either by a thermal process or by deposition. Advantageously, steps e) and/or g) can be a wet or dry etching step. This is a more economic material removal step than the edge polishing and double-sided polished step as used in the prior art.
Preferably, steps e) and g) can be performed using a non-selective and/or isotropic etching solution. Using the non-selective etching process, both the dielectric layer and the underlying part of the donor substrate, for instance a semiconductor material, can be removed in one step. By using an isotropic etching solution, the process is furthermore simplified as varying etching rates, depending on the crystallographic directions that are different in the chamfered region compared to the rest of the substrate, do not have to be taken into account. This simplifies the control of the process.
The dielectric material removal step, independent of whether the dielectric layer is removed from the donor and/or the handle substrate (in case a dielectric is also present on the handle substrate), does not have an impact on the quality of the buried dielectric layer as this one is sandwiched between donor and handle substrate.
Advantageously, during step d), a layer with a thickness of about 100 Å to 10,000 Å, in particular 1000 Å to 3000 Å, can be removed in the non-attached region of the donor substrate. Compared to the double-sided polishing step mandatory in the prior art during which a large amount of material, namely in the order of 5 μm on each side, had to be removed to get rid of the protruding portions, the material removal in the process according the invention can be less so that one donor substrate can be reused more often, for instance, more than ten times, compared to the prior art process. This is also made possible by the fact that the reclaimed donor substrate remains within the semi-standard concerning the substrate dimensions.
Preferably, steps d) and/or g) can be carried out at a temperature of less than 500° C., preferably at less than 350° C. Thus, the whole process until detachment is carried out at low temperature so that no detachment can occur in the non-attached regions which could lead to unwanted particle contamination of the final product.
According to a preferred embodiment, the surface treatment step can, at most, comprise a polishing step, in particular a CMP polishing and a cleaning step before and/or after the polishing step. Thus, compared to the prior art, a greatly simplified reclaiming process can be carried out. The polishing step is used to get the desired surface quality of the reclaimed remainder of the donor substrate, typically a mirror-polished quality, and the cleaning steps, as known in the art, make the reclaimed donor substrate ready for reuse. Thus, instead of a three-level reclaiming process: i) edge polishing, ii) double-sided polishing, iii) mirror polishing plus the various cleaning steps, the invention provides the possibility to carry out the reclaiming process with only one simple mirror polishing step.
In this context, the term “about the thickness h” relates to thickness that are such that the remainder can be planarized using only a CMP process. This means that a layer is removed in step d) of claim 1 that has a thickness of at least h minus about 50 nm. This would lead to a protruding region in the remainder of the donor substrate with a height of about 50 nm, which can be dealt with by a simple CMP polishing step.
Advantageously, during CMP polishing, a layer of less than 3 μm, preferably less than 1 μm, can be removed from the surface where detachment occurred. As mentioned above, this limited material removal brings the advantage that one donor substrate can be reused more often, in particular, more than ten times, in the layer transfer process.
Advantageous embodiments will be described in combination with the enclosed Figures.